The Fischer-Tropsch synthesis (hereinafter referred to as “F-T”) is a chemical reaction process for producing liquid fuels from syngas (H2+CO) in the presence of metal catalyst under appropriate conditions. Since it was first discovered in Germany by Franz Fischer and Hans Tropsch in 1923, the Fischer-Tropsch synthesis process has undergone a significant development over 80 years in many respects such as catalyst, reactor and their corresponding reaction process.
Iron-based and cobalt-based catalysts are two major types of catalysts for the Fischer-Tropsch synthesis process, and the iron-based catalyst is divided into three categories for different reactions and process systems, including a precipitated iron catalyst, a supported iron catalyst, and a fused iron catalyst, respectively.
Fischer-Tropsch synthesis reactors have been developed from tubular fixed bed reactors to fluidized reactors (fixed fluidized beds and circulating fluidized beds), until recently the most advanced slurry bed reactors. Compared with other processes, the Fischer-Tropsch synthesis process in the slurry bed reactors has the following advantages:    (1) Ability to using coal-based syngas with low H2/CO ratio produced in modern coal gasifiers as feed gas, and reduction of coal-based syngas transformation load.    (2) Excellent heat exchange efficiency, good performance in temperature control.    (3) Simple construction, lower costs, running feasibility under high space velocity of feed gas, and significantly higher production efficiency than that of the fixed bed reactor.    (4) Easy loading and unloading of catalyst which can be reduced or supplementarily added in-situ, and significant improvement in efficient run time.
However, the current slurry bed process is normally operated at 230-250° C. and about 25% of the syngas energy is released in the form of heat in this reaction process. Therefore, a large amount of low-grade steam of 0.8-1.0 MPa is produced if the process is operated at a lower temperature, and such steam is difficult to be utilized, which leads to a total energy conversion efficiency of only 38-41%. Therefore, it is necessary to increase the operating temperature of the slurry bed in the Fischer-Tropsch synthesis, thus the total energy conversion efficiency can be further improved in this process.
However, the high-temperature Fischer-Tropsch synthesis is generally achieved in the high temperature fluidized bed reactor with a fused iron catalyst. The catalyst used for the high temperature Fischer-Tropsch synthesis in Sasol Company is a fused iron catalyst, in which magnetite (its main component is Fe3O4) is used as the main raw material, melted at about 1500° C. and then added promoters such as K2O, CaO or Al2O3. The specific surface area of the catalyst is low, but its high strength is suitable for high-temperature operation. Another kind of the high-temperature Fischer-Tropsch synthesis catalyst is prepared by co-precipitation, such as a Fe—Cu—K catalyst disclosed in U.S. Pat. No. 6,844,370 by Sasol Company, and a precipitated Fe—Cu—Cr—K—Na catalyst disclosed in several China patents of CN1695803A, CN1695804A, and CN1817451A by Yanzhou Mining Group Co., Ltd. All of the above catalysts are suitable for a high temperature fluidized bed reactor. The selectivity data published have shown that methane is less than 10%, and C23+ hydrocarbons are more than 65%; and the heavy hydrocarbons are obviously increased, compared with that of the high temperature fused iron catalyst.
The Fischer-Tropsch synthesis operated in a high temperature slurry bed reactor requires that the catalyst has sufficiently high mechanical strength to withstand high temperature, high gas flow rate, high amount of steam generated (for the iron-based catalyst), and other unfavorable factors under three-phase reaction conditions. Patent CN1213802C jointly applied by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and China Petroleum & Chemical Corporation disclosed a kind of two-active-component supported iron-based catalyst, which is prepared only in a laboratory small scale. The catalyst can be operated in a slurry bed reactor for syngas to selectively synthesize gasoline and diesel oil fractions (<C20) and the reactive temperature can be up to 300° C., but the preparation process is complex and difficult to achieve a scale-up of process.
The main difficulties of catalyst being operated in the high temperature slurry bed are that the iron-based catalyst will produce complex thermo-chemical strain, which will result in severe chemical and physical abrasion. Meanwhile, both desorption and dissociation ability of the catalyst for CO and H2 will change significantly at a higher temperature. These problems will affect the stable operation of the catalyst in the high temperature slurry bed.
The present inventors have synthesized a kind of catalyst after a lot of research work, using an advanced catalyst design method and a new catalyst preparation process with co-precipitation as the main step. The produced catalyst for the Fischer-Tropsch synthesis in a slurry bed reactor has the following advantages: high strength, suitability for high temperature operation, and significant tendency in hydrocarbon products selectivity to those hydrocarbons with a medium-carbon number distribution. Also, the active component content in the catalyst is in a high level, uniformly dispersed, high active and stable; and the catalyst has high anti-abrasion strength and better distribution of hydrocarbon products than that of the low temperature process, and improvement in the total energy conversion efficiency for the Fischer-Tropsch synthesis process; especially it is suitable for operating at a higher temperature (250-300° C.), and thus obtains a higher grade of steam.